A common process for manufacturing lower olefins is through pyrolytic cracking of saturated hydrocarbon feedstocks containing hydrocarbons such as ethane, propane, butane, pentane, and crude oil fractions such as naphtha and gas oil. Producers of lower olefins are always looking for lower cost hydrocarbon feedstocks that can be economically upgraded by pyrolytic cracking processes to lower olefins. Lower cost materials that are of interest for the conversion to a lower olefins product are any paraffinic hydrocarbon material that contains high boiling point or non-vaporizable coke precursors such as crude oil and fractions of crude oil, such as petroleum residuum. While crude oil and petroleum residuum are attractive from a cost standpoint, they do not make good feedstocks for pyrolytic cracking, because they do not completely vaporize in the convection section of traditional pyrolytic cracking furnaces.
A recent advance in pyrolysis of crude oil and crude oil fractions containing pitch is shown in U.S. Pat. No. 6,632,351. In the '351 process a crude oil feedstock or crude oil fraction(s) containing pitch is fed directly into a pyrolysis furnace. The process comprises feeding the crude oil or crude oil fractions containing pitch to a first stage preheater within a convection zone, where the crude oil or crude oil fractions containing pitch are heated within the first stage preheater to an exit temperature of at least 375° C. to produce a heated gas-liquid mixture. The mixture is withdrawn from the first stage preheater, steam is added and the gas-liquid mixture is fed to a vapor-liquid separator, followed by separating and removing the gas from the liquid in the vapor-liquid separator, and feeding the removed gas to a second preheater provided in the convection zone. The preheated gas is then introduced into a radiant zone within the pyrolysis furnace, and pyrolyzed to olefins and associated by-products. While this is an improvement in the overall process, there are still limitations in achieving higher yields of more valuable products due to coke formation in the convection section and vapor-liquid separator at increased separation temperatures needed to increase hydrocarbon gas feed rates to the radiant section of the furnace where pyrolysis takes place.
U.S. Pat. No. 4,264,432 discloses a process and system for vaporizing heavy gas oil prior to thermal cracking to olefins, by flashing with steam in a first mixer, superheating the vapor, and flashing in a second mixer the liquid from the first mixer. Such a process is apparently directed to merely vaporization of heavy gas oils having an end point of about 1005° prior to pyrolysis cracking of the heavy oil, and is not directed to creating an acceptable pyrolysis feedstock from an otherwise unacceptable feedstock having undesirable coke precursors and/or high boiling pitch fractions.
What is needed is an improved process that permits the economical processing of a heavy hydrocarbon feedstock to produce lower olefins in higher yield, without causing unacceptable fouling or coking in the convection section or the vapor-liquid separation equipment.